In this study we showed that antigen-specific lymphocyte proliferation and interleukin-2 production by peripheral blood lymphocytes from patients under thiopental anesthesia are significantly depressed. We have also shown that tetanus toxoid specific CD4+ T cell clones, with a known cytokine production profile, were sensitive to the inhibitory effects of thiopental and exhibited decreased proliferation to tetanus toxoid as well as decreased secretion of interleukin-2. We observed no difference regarding interleukin-4 production by these clones. The data suggest that the immunosuppressive effect of thiopental is confined to antigen-specific responses. In addition, we have shown that interleukin-2 production is not significantly altered. This last finding has important implications regarding the type of immune response that is most affected by this anesthetic agent. In spite of the transient decrease in antigen-driven interleukin-2 synthesis, no clinical evidence of infection was noted in any of these patients.